Q.Integrate the function ax+b
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — U Substitution
U Substitution: The Reverse Chain Rule
The chain rule differentiates composite functions: the derivative of sin(x2) is cos(x2)⋅2x — differentiate the outer function, then multiply by the derivative of the inside. Integration asks the reverse: given cos(x2)⋅2x, find the original function. That's what u substitution does — it reverses the chain rule.
The Core Intuition
When an integral looks like "a function times the derivative of its inside," substitute the inside with u and the derivative of the inside with du. Consider:
∫2xcos(x2)dx
Here 2x is the derivative of x2, and x2 is the inside of cos(x2). Let u=x2, so du=2xdx:
∫cos(u)du=sin(u)+C=sin(x2)+C
Check: the derivative of sin(x2) is cos(x2)⋅2x.
The Precise Statement
∫f(g(x))⋅g′(x)dx=∫f(u)duwhere u=g(x),du=g′(x)dx
Valid provided g is differentiable and the resulting integral in u is simpler.
The Step-by-Step Method
- Identify a function g(x) whose derivative g′(x) also appears (possibly up to a constant factor).
- Set u=g(x), compute du=g′(x)dx.
- Rewrite the entire integral in u and du — every x and dx must be replaced.
- Integrate with respect to u.
- Substitute back u=g(x).
You cannot mix variables. If any x remains after substitution, you chose the wrong u (or must solve for x in terms of u — rare).
A Second Example (with a constant factor)
Evaluate ∫xx2+1dx. Let u=x2+1, so xdx=21du:
∫u⋅21du=21⋅32u3/2+C=31(x2+1)3/2+C
When Does It Work?
When the integrand is something times the derivative of something inside. Common patterns:
- x⋅f(x2) — derivative of x2 is 2x, so u=x2
- eg(x)⋅g′(x) — derivative of g(x) appears
- g(x)g′(x) — leads to log∣g(x)∣ …
Concept: U Substitution — we set u equal to the expression inside the square root to simplify the integral.
Let u=ax+b. Then du=adx, so dx=adu.
The integral becomes:
∫u⋅adu=a1∫u1/2du
Integrate: …
The key idea is to use the substitution u=ax+b, which simplifies the square root into a power of u. The integral becomes 3a2(ax+b)3/2+C.
Why U-Substitution Works Here
When you see a composite function like ax+b, your first instinct should be to look for an "inner function" that you can replace with a single variable. The square root is an "outer" operation applied to the linear expression ax+b. If we let u=ax+b, then the square root becomes u, which is just u1/2 — a simple power function we know how to integrate.
The real magic is that the derivative of ax+b is a constant a, so du=adx gives us a clean way to replace dx as well. No messy chain rule to untangle — just a straight substitution.
Always check if the derivative of your chosen u appears (up to a constant factor) in the original integral. Here, du=adx means we can solve for dx=adu, and the constant a just factors out.
Step-by-Step Solution
1. Choose the substitution.
Let u=ax+b. This is the expression inside the square root. The entire integral is ∫ax+bdx, so replacing ax+b with u gives ∫udx — but we still have dx, not du.
2. Find du in terms of dx.
Differentiate u with respect to x:
dxdu=a
So du=adx, which means dx=adu.
3. Rewrite the integral entirely in u.
Substitute dx and the square root:
∫ax+bdx=∫u⋅adu=a1∫u1/2du …
Method: Substitution for a Linear Inner Function
Use this whenever a single linear expression ax+b sits inside another operation (a root, a power, an exponential): replace the whole linear chunk with one new variable.
Steps
Step 1: Set u equal to the linear expression.
Let u=ax+b. Then du=adx, so dx=adu. Because the derivative of a linear expression is the constant a, the swap is clean — no leftover x terms remain.
Step 2: Rewrite the whole integral in u. …
Common Mistakes
Mistake 1: Forgetting the a1 factor from dx=adu.
Why it's wrong: substitution changes dx, and skipping the a1 scales the whole answer wrongly. Correct approach: always compute du and solve for dx before substituting.
Mistake 2: Leaving the answer in terms of u. …
Showing the 12 most recent of 51 on this concept.
- AP EAPCET 2023Set eng-2023-05-17-AN1 markMCQQ.If ∫1+x2x3dx=A(1+x2)3/2+B(1+x2)1/2+C, then A+B= (A) 2/3 (B) −2/3 (C) 1/3 (D) −1/3
›Reveal solutionSolution
The substitution u=1+x2 turns the integral into a simple power-rule computation, giving A=1/3 and B=−1, so A+B=−2/3.
Concept and Intuition
Whenever the integrand has an odd power of x alongside a function of x2 (here 1+x2), substituting u=1+x2 (so du=2xdx) converts the odd-power part into a polynomial in u, making the integral elementary.
Step-by-Step Solution
- Let u=1+x2, du=2xdx, and x2=u−1.
- x3dx=x2⋅xdx=(u−1)⋅2du.
- ∫1+x2x3dx=∫u(u−1)⋅2du=21∫(u1/2−u−1/2)du.
- =21(32u3/2−2u1/2)+C=31u3/2−u1/2+C. …
- AP EAPCET 2022Set eng-2022-07-07-AN1 markMCQQ.∫(1+x)2022dx= (A) (1+x)20212[20201+x−20211]+C (B) (1+x)20222[20201+x−2021x]+C (C) (1+x)2[2022(1+x)2022−2021(1+x)2021]+C (D) (1+x)21[(1+x)10101−(1+x)10111]+C
›Reveal solutionSolution
Substituting t=1+x turns the integral into a simple power-rule integral in t; back-substituting and factoring reproduces option (A)'s bracketed form.
Concept and Intuition
Whenever an integrand is a function purely of 1+x, the substitution t=1+x (so x=t−1, x=(t−1)2) turns the messy radical expression into a clean power of t, and the pieces of dx that are left over (2(t−1)dt) combine with the t−2022 factor to give a difference of two pure power terms — each integrable by the ordinary power rule.
Step-by-Step Solution
- Let t=1+x. Then x=t−1, x=(t−1)2, and dx=2(t−1)dt.
- The integral becomes ∫t20222(t−1)dt=2∫(t−2021−t−2022)dt.
- Integrate termwise: 2∫t−2021dt=−20202t−2020, and −2∫t−2022dt=−20212⋅(−1)t−2021⋅(−1), combining to 20212t−2021−20202t−2020. …
- AP EAPCET 2021Set eng-2021-08-24-FN1 markMCQQ.If ∫3x{1+3x4}1/7dx=A(1+3x4)B+C, then value of AB= ____ (A) 23 (B) 43 (C) 323 (D) 34
›Reveal solutionSolution
A direct substitution u=1+x4/3 turns the integral into a simple power rule, from which A and B are read off and multiplied.
Concept and Intuition
When the integrand contains x1/3 times a function of x4/3, substituting u=1+x4/3 (whose derivative involves exactly x1/3dx) is the natural simplification.
Step-by-Step Solution
- Let u=1+x4/3. Then du=34x1/3dx⇒x1/3dx=43du.
- The integral ∫x1/3(1+x4/3)1/7dx=43∫u1/7du.
- ∫u1/7du=8/7u8/7=87u8/7.
- So the integral =43⋅87u8/7+C=3221(1+x4/3)8/7+C. …
- AP EAPCET 2022Set eng-2022-07-07-AN1 markMCQQ.∫x+x2+2dx= (A) 23(x+x+2)3/2−2(x+x2+2)1/4+C (B) 31(x+x2+2)3/2−2(x+x2+2)1/4+C (C) (x+x2+2)−3/2−2(x+x2+2)−1/2+C (D) 3x+x2+2(x+x2+2)2−6+C
›Reveal solutionSolution
The substitution t=x+x2+2 rationalises the nested radical; the resulting antiderivative, written as a single fraction, matches option (D). Answer: option (D).
Concept and Intuition
Integrals containing x+x2+a2 are a classic signal to substitute t equal to that whole expression — it converts the awkward nested square root into simple powers of t, because x and x2+a2 can both be written as clean rational/linear functions of t.
Step-by-Step Solution
- Let t=x+x2+2. Then x2+2=t−x; squaring, x2+2=t2−2tx+x2⇒2=t2−2tx⇒x=2tt2−2.
- Then x2+2=t−x=t−2tt2−2=2t2t2−t2+2=2tt2+2.
- Differentiate t w.r.t. x: dxdt=1+x2+2x=x2+2x2+2+x=x2+2t=(t2+2)/(2t)t=t2+22t2, so dx=2t2t2+2dt.
- Substitute into the integral: ∫tdx=∫t1/2⋅2t2t2+2dt=21∫(t1/2+2t−3/2)dt.
- Integrate: 21[32t3/2−4t−1/2]+C=31t3/2−2t−1/2+C. …
- AP EAPCET 2021Set eng-2021-08-25-AN1 markMCQQ.∫cosxdx= (A) 2xsinx+2cosx+c (B) 2xsinx+2sinx+c (C) 2xsinx−2cosx+c (D) xcosx−2sinx+c
›Reveal solutionSolution
Substitute t=x to turn the integral into a standard integration-by-parts problem. Answer: 2xsinx+2cosx+c.
Concept and Intuition
Whenever you see x trapped inside a trig or exponential function, substituting t=x converts it into a polynomial-times-trig integral solvable by parts.
Step-by-Step Solution
- Let t=x⇒x=t2, dx=2tdt.
- ∫cosxdx=∫cost⋅2tdt=2∫tcostdt.
- Integrate by parts: ∫tcostdt=tsint−∫sintdt=tsint+cost. …
- AP EAPCET 2022Set eng-2022-07-08-FN1 markMCQQ.0<x<1, ∫x2−x5dx=31log∣f(x)∣+C, then f(1/2)= (A) 8+78−7 (B) 8−78+7 (C) 2(8−7) (D) 2(8−7)2
›Reveal solutionSolution
Substituting t=x3 then 1−t=w2 integrates ∫dx/(x1−x3) cleanly to 31log1+1−x31−1−x3, and evaluating at x=1/2 gives 8+78−7.
Concept and Intuition
The key simplification is x2−x5=x2(1−x3), since 0<x<1 makes x>0 so x2(1−x3)=x1−x3. From there, the substitution t=x3 turns the integral into the very standard form ∫t1−tdt, solvable by a further substitution 1−t=w2.
Step-by-Step Solution
- Rewrite: I=∫x2−x5dx=∫x1−x3dx (using x>0).
- Let t=x3⇒dt=3x2dx⇒dx=3x2dt. Then I=∫3x2⋅x1−tdt=∫3x31−tdt=31∫t1−tdt (since x3=t).
- Let 1−t=w2⇒t=1−w2, dt=−2wdw: ∫t1−tdt=∫(1−w2)w−2wdw=−2∫1−w2dw=−log1−w1+w=log1+w1−w.
- So I=31log1+w1−w+C where w=1−t=1−x3, i.e. f(x)=1+1−x31−1−x3.
- Verify by differentiating (chain rule through w) that this reproduces x1−x31 — confirmed. …
- AP EAPCET 2025Set eng-2025-05-22-FN1 markMCQQ.∫x2x4+x2+1x4−1dx= (A) x2x4+x2+1+c (B) xx4+x2+1+c (C) 2xx4+x2+1+c (D) x4x4+x2+1+c
›Reveal solutionSolution
Differentiating the candidate xx4+x2+1 reproduces the given integrand exactly, confirming it as the antiderivative.
Concept and Intuition
When an integrand looks like it could come from a quotient rule (a square root over a power of x), it is often faster to differentiate a plausible candidate of that shape and check, rather than search for a substitution from scratch.
Step-by-Step Solution
- Try g(x)=xx4+x2+1=xN where N=x4+x2+1.
- N′=2x4+x2+14x3+2x=Nx(2x2+1).
- Quotient rule: g′(x)=x2N′x−N=x2Nx2(2x2+1)−N=Nx2x2(2x2+1)−N2.
- N2=x4+x2+1, so the numerator is x2(2x2+1)−(x4+x2+1)=2x4+x2−x4−x2−1=x4−1.
- So g′(x)=x2x4+x2+1x4−1 — exactly the given integrand. …
- AP EAPCET 2025Set eng-2025-05-27-FN1 markMCQQ.∫(1+x)x−x2dx= (A) −21−x1+x+c (B) −1+x1−x+c (C) −21+x1−x+c (D) 21−x1+x+c
›Reveal solutionSolution
Substituting t=x turns the surd-heavy integrand into ∫(1+t)3/2(1−t)1/22dt, whose antiderivative is exactly −21+t1−t.
Concept and Intuition
When an integrand mixes x and x−x2=x1−x, substituting t=x clears every square root of x at once, converting the whole thing into a rational-power integral in t that matches the derivative of 1+t1−t — a standard "recognise the derivative" pattern worth memorising for CET-style problems.
Step-by-Step Solution
- Write x−x2=x(1−x)=x1−x, so the integral is
I=∫(1+x)x1−xdx.
- Let t=x, so x=t2, dx=2tdt:
I=∫(1+t)⋅t⋅1−t22tdt=∫(1+t)(1−t)(1+t)2dt=∫(1+t)3/2(1−t)1/22dt.
- Let y=1+t1−t. Differentiating y2=1+t1−t: 2yy′=(1+t)2−(1+t)−(1−t)=(1+t)2−2 ⇒ y′=y(1+t)2−1=(1+t)3/2(1−t)1/2−1. …
- AP EAPCET 2026Set eng-2026-05-14-FN1 markMCQQ.∫(x5+1)6/5dx= (A) 5x5+11+c (B) x5x5+1+c (C) 5x5+1x+c (D) 5x5+1+c
›Reveal solutionSolution
Recognizing the integrand as the derivative of 5x5+1x (verified by direct differentiation) gives the antiderivative immediately.
Concept and Intuition
For integrals of the form ∫(xn+1)(n+1)/ndx, a useful trick is to guess that the antiderivative looks like (xn+1)1/nx (a ratio designed to make the product-rule differentiation collapse nicely), and then verify by differentiating it — if it reproduces the integrand exactly, we're done. This is often faster than a substitution for this particular family.
Step-by-Step Solution
- Guess the antiderivative g(x)=(x5+1)1/5x=x(x5+1)−1/5.
- Differentiate using the product rule: g′(x)=(x5+1)−1/5+x⋅(−51)(x5+1)−6/5⋅5x4.
- Simplify the second term: x⋅(−51)(5x4)(x5+1)−6/5=−x5(x5+1)−6/5.
- So g′(x)=(x5+1)−1/5−x5(x5+1)−6/5.
- Factor out (x5+1)−6/5: g′(x)=(x5+1)−6/5[(x5+1)−x5]=(x5+1)−6/5×1=(x5+1)−6/5. …
- AP EAPCET 2026Set eng-2026-05-14-FN1 markMCQQ.If ∫(x+5)x−5dx=152x−5f(x)+c, then f(6)= (A) 5 (B) 20 (C) 100 (D) 53
›Reveal solutionSolution
Substituting u=x−5 turns the integral into a simple power-rule computation; matching it to the given form and evaluating f at x=6 gives 53.
Concept and Intuition
An integrand with x−a multiplying a linear expression in x is best handled by substituting u=x−a, which converts everything into pure powers of u that integrate directly via the power rule. After integrating, we factor the result to match the given answer template and read off f(x).
Step-by-Step Solution
- Let u=x−5, so x=u+5, and x+5=u+10. Also dx=du.
- The integral becomes ∫(u+10)udu=∫(u3/2+10u1/2)du.
- Integrate term by term: ∫u3/2du=52u5/2, and ∫10u1/2du=10×32u3/2=320u3/2.
- So the integral =52u5/2+320u3/2+c.
- Factor out 152u3/2 (the common factor, chosen to match the given form's 152): 52u5/2=152u3/2(3u) and 320u3/2=152u3/2(50). So the integral =152u3/2(3u+50)+c=152u1/2⋅u(3u+50)+c. …
- AP EAPCET 2022Set eng-2022-07-05-FN1 markMCQQ.∫(x−3)4/5(x+1)6/5dx= (A) 455x+1x−3+C (B) 45(x−3x+1)1/5+C (C) 51(x+1x−3)1/5+C (D) 45(x+4x−3)4/5+C
›Reveal solutionSolution
Splitting off (x+1)2 turns the integrand into a pure power of t=x+1x−3, giving 45(x+1x−3)1/5+C.
Concept and Intuition
When an integrand has the form (x−a)p(x−b)q with p+q an integer (here 54+56=2), factoring out (x−b)p+q and substituting t=x−bx−a collapses the whole thing to a simple power of t — a standard trick for these "unequal fractional exponent" integrals.
Step-by-Step Solution
- (x−3)4/5(x+1)6/5=(x+1)2[x+1x−3]4/5 (factoring out (x+1)4/5+6/5=(x+1)2).
- So the integrand is (x+1)−2[x+1x−3]−4/5.
- Let t=x+1x−3. Then dxdt=(x+1)2(x+1)−(x−3)=(x+1)24, so (x+1)−2dx=4dt.
- The integral becomes 41∫t−4/5dt=41⋅1/5t1/5+C=45t1/5+C. …
- AP EAPCET 2021Set eng-2021-08-23-AN1 markMCQQ.∫2/e1/ex(logx)1/31dx= (A) 23{1+(log(2)−1)2/3} (B) 1 (C) 23{1+(log(2)+1)3/2} (D) 23{1−(log(2)−1)2/3}
›Reveal solutionSolution
Substitute u=logx to turn the integral into ∫u−1/3du, then evaluate between the transformed limits u=log2−1 and u=−1.
Concept and Intuition
An integrand of the form x⋅g(logx)1 always calls for the substitution u=logx, since du=dx/x removes the x and 1/x entirely, leaving a pure power of u.
Step-by-Step Solution
- Let u=logx, so du=dx/x.
- Limits: at x=2/e, u=log(2/e)=log2−1. At x=1/e, u=log(1/e)=−1.
- The integral becomes ∫log2−1−1u−1/3du=[2/3u2/3]log2−1−1=23[u2/3]log2−1−1.
- Using the real cube root, (−1)2/3=((−1)1/3)2=(−1)2=1.
- So the value is 23[1−(log2−1)2/3].
Common Mistakes …
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